This invention relates to a tire wheel having a hump which prevents dislodgment of a bead portion of a pneumatic tire from a bead seat of the tire wheel onto its well when inner pressure of the tire lowers.
A tire wheel having a hump has been known as shown, for example, in FIG. 1. The hump 62 of the tire wheel 61 has a seat side inclined surface 65 on a side of a bead seat 64 and a well side inclined surface 67 on a side of a well 66 on both sides of a top 63. The seat side inclined surface 65 is formed by a circular arc surface which is radially outwardly convex having a radius Rd, a center D and a circular arc surface which is radially inwardly convex having a radius Re and a center E. On the other hand, the well side inclined surface 67 is also formed by a circular arc surface which is radially outwardly convex having the radius Rd and the center D and a circular arc surface which is radially inwardly convex having a radius Rf and a center F.
With this tire wheel, however, as the seat side inclined surface 65 of the hump 62 consists of two circular arc surfaces smoothly changing, there is a risk of a bead portion B of a pneumatic tire T easily riding over the hump 62 to fall onto the well 66 when inner pressure of the tire has lowered due to a puncture or the like and great transverse forces act on the pneumatic tire upon turning.
In order to solve this problem, tire wheels have been proposed as shown in FIGS. 2 and 3. In the tire wheel 76 shown in FIG. 2, a cross-sectional shape of a hump 77 is changed with circumferential positions. Starting from the shape shown in solid lines at a circumferential position of 0.degree., shapes at circumferential positions of 90.degree. and 270.degree. are as shown in a broken line and a shape at a circumferential position of 180.degree. is as shown in a two-dot-and-dash line. In other words, an inclined angle of a plane P is progressively changed dependent upon change in circumferential position about an axial determined position K. A seat side inclined surface 78 is inclined at a large angle with respect to a tire axis at the circumferential position of 0.degree., but at a small angle at the circumferential position of 180.degree..
On the other hand, with the tire wheel 81 shown in FIG. 3, a seat side inclined surface 83 of a hump 82 extends radially outwardly or substantially perpendicularly to a tire axis.
It is assumed that inner pressure of a tire T mounted on the tire wheel 76 shown in FIG. 2 lowers and the tire T is subjected to traverse forces in a direction shown by an arrow in FIG. 4. Under such an assumption, as the inclined angle of the seat side inclined surface 78 changes with circumferential positions, it is difficult for a bead portion B of the tire T to move toward the well 79 at positions near to the circumferential position of 0.degree. where the inclined angle of the seat side inclined surface 78 is large because the bead portion B abuts against the inclined surface 78. However, it is easy for the bead portion B to move toward the well 79 at positions near to the circumferential position of 180.degree. because the inclined angle of the seat side inclined surface 78 is small sufficient to easily ride over the inclined surface 78.
After part of the bead portion B (near to the 180.degree. circumferential position) has once deformed and ridden on the seat side inclined surface 78 in this manner, the deformed portion of the bead portion B is repeatedly subjected to traverse forces every time it arrives on a ground contacting side so that the deformed portion progressively moves axially inwardly until the bead portion B falls onto the well 79. Although the tire wheel 76 shown in FIG. 2 is improved to some extent for preventing the bead portion B from falling onto the well 79 in comparison with the tire wheel 61 shown in FIG. 1, the tire wheel 76 is insufficient to be used for a high performance tire recently developed. Moreover, the tire wheel is particular in shape so that its forming is difficult and expensive.
On the other hand, it is also assumed that when inner pressure of the pneumatic tire T mounted on the tire wheel 81 shown in FIG. 3 lowers, traverse forces act upon the tire T. In this case a bead toe C of the bead portion B on a ground contacting side abuts against the seat side inclined surface 83 of the hump 82 extending substantially vertical to the tire axis. Therefore, large shearing forces act upon the bead toe C to damage part of the bead toe C and to expose carcass cords and the like. Moreover, there is a problem that such damage of the bead toe C would also occur when the tire T is removed from the wheel because a fairly large force is required for the removal.